Is it possible to convert the sheet to multilangual? I read that you can create multilangual sheets, not sure how much work it would be to change it afterwards.

I'd like to do the German version, but I can either copy the sheet to a new one (I've never created a sheet für Roll20, but changing some text cant be that difficult), but then there would be two versions.Every update /change would have to be done twice.

Or somebody who knows how to do it makes a multilanguage version. I allready translated parts of other sheets, it's easy doing.

Roll20 has a built in system for translating, which it sounds like you already know of. It's relatively simple to set up as far as I can tell. Adding it to the existing one shouldn't be too hard, tho might be quite tedious. Still definitely better than making a new, German only sheet. If need be I might be able to help, but I'm presently somewhat tapped out so it wouldn't likely be for a while.